This invention relates to electrochemical cell stacks, particularly, to monopolar filter press cell stacks, and more particularly to internally pressurized monopolar water electrolytic cells for the production of hydrogen and oxygen.
Electrosynthesis is one example of an electrochemical process comprising a method for the production of chemical reaction(s) that is electrically driven by passage of an electric current, typically a direct current (DC), in an electrochemical cell through an electrolyte between an anode electrode and a cathode electrode from an external power source. The rate of production is proportional to the current flow in the absence of parasitic reactions. For example, in a liquid alkaline water electrolysis cell, the DC current is passed between the two electrodes in an aqueous electrolyte to split water, the reactant, into component product gases, namely, hydrogen and oxygen where the product gases evolve at the surfaces of the respective electrodes.
Water electrolysers have typically relied on membranes or separators between the two halves of an electrolysis cell to ensure that the two gases, namely, oxygen and hydrogen produced in the electrolytic reaction are kept separate and do not mix. Each of the separated gases must be discharged from the cell at essentially the same pressure since membranes and separators fail with pressure differential. Simple pressure control systems, such as a small water column of several centimeters in height for each gas and discharge to atmospheric pressure are used to control the pressure within this pressure differential.
In the conventional monopolar cell design in wide commercial use today, one cell or an array of cells in parallel is contained within one functional electrolyser, cell compartment, or individual tank. Each cell is made up of an assembly of electrode pairs in a separate tank where each assembly of electrode pairs connected in parallel acts as a single electrode pair. The connection to the cell is through a limited area contact using an interconnecting bus bar such as that disclosed in Canadian Patent No. 3,02,737, issued to A. T. Stuart (1930). The current in the form of a flow of electrons is taken from the cathode bus bar via an electrical connection to a portion of a cathode in one cell, then through the electrolyte in the form of ions to the anode of that cell and then to the anode bus bar using a similar electrical connection. The current is usually taken off one electrode at several points and the connection made by means of clamps, soldered joints, mechanical screw connections and the like.
Electrolysis apparatus having pressurized external cell structures are known for producing hydrogen. For example, U.S. Pat. No. 5,665,211, issued 1997, describes a pressurized container within which is an electrolytic cell. There is no integration of the cell itself as the pressure containment device, and, thus, the apparatus is bulky and heavy. U.S. Pat. No. 3,652,431, issued 1972, describes an electrolysis cell where external pressure from a liquid such as water is used to support a container in which pressurized electrolysis is conducted. U.S. Pat. No. 4,042,481, issued 1977, describes a pressurized tank containing cylindrical porous anode and cathode tubes which allow escape of the oxygen and hydrogen produced. However, the apparatus requires the need for a tank to house cells and, thus, this does not represent efficient use of overall space or footprint. There is also the potential for mixing of oxygen and hydrogen produced if gas does not diffuse through the porous electrode tubes. The cylindrical configuration of the anodes and cathodes present fabrication challenges and the spacing of these electrodes will require substantial room to prevent non-uniform currents if multiple cells are used. U.S. Pat. No. 5,733,422, issued 1998, describes a tank with a header box wherein the top is screwed onto the side wall plates. Again, this is clearly not a design suitable for lightweight and inexpensive polymeric materials.
There is, therefore, a need for electrolytic cells, particularly, water electrolysers, which do not suffer from the aforesaid disadvantages.
The present invention provides an electrolytic cell stack having a beneficial novel relationship of cell components involving the inverse structural role of some components, through the use of a single electrolyte circulation and membrane frame within each cell. There is no need for end-boxes and compressible elastomeric materials which, however, may be optional. The circulation and membrane frame preferably, is formed of a structural plastics material, which can provide support to thin foil electrodes if the latter are used. In the absence of gaskets and compressible elastomeric frames, advantageous higher operating temperatures can be readily attained. This is particularly so when the cell stack is pressurized as hereinafter described.
In one aspect, the invention provides an electrochemical cell stack comprising stack walls and a plurality of electrolytic cells within the stack walls, each cell comprising cell members selected from an anode; a cathode; a membrane separator frame formed of a non-conductive material and having a first side and a second side opposite to said first side;
(a) a frame first planar peripheral surface on said first side;
(b) a frame second planar peripheral surface on said second side; and
(c) a central portion defining a membrane-receiving aperture;
a membrane within the aperture which provides an anolyte circulation chamber and a catholyte circulation chamber distinct one from the other within said frame; an impermeable cell end wall formed of a non-conductive material between said anode and said cathodes and the anodes and cathodes of adjacent cells of the stack; wherein each of said anode, said cathode, said separator frame and said end wall has a portion defining an anolyte flow inlet channel, a catholyte flow inlet channel, a spent anolyte channel and a spent catholyte channel; and wherein said anolyte flow inlet channel and said spent anolyte channel are in communication with said anolyte circulation chamber, and said catholyte flow inlet channel and said spent catholyte channel are in communication with said catholyte circulation chamber.
In one preferred embodiment, the invention provides a cell stack as hereinabove defined wherein said anode has an anode first planar surface which abuts said frame first planar peripheral surface as to define with said member said anolyte circulation chamber within the confines of said frame, and said cathode has an cathode second planar surface which abuts said frame second planar peripheral surface as to define with said member said catholyte circulation chamber within the confines of said frame.
In another preferred embodiment, the invention provides a cell stack as hereinabove defined wherein said anode in whole or in part is disposed within said anolyte circulation chamber and said cathode in whole or in part is disposed within said catholyte circulation chamber. In this later defined cell stack, one or both of the anode and the cathode are in contact with the membrane, on opposite sides thereof, within the respective electrolyte circulation chamber, as for example, a laminate with or coating on the membrane.
The production of a bilayer or trilayer porous assembly offers the distinct advantages of minimal electrode/membrane thickness and, hence, inter-electrode cell resistance, as well as ease of processing on a continuous basis by the integration of separate parts, namely, current carrier+activation+membrane, using known, suitable processing methods.
The production of such a bi or trilayer composite structure can be carried out, for example, by utilizing a core membrane material and metallizing this externally, wherein the membrane may be either polymeric or ceramic in nature, formed by, for example, weaving, felting, tape casting, sintering and the like. The metallizing process can be selected, but not limited to one of plasma vapour deposition, chemical vapour deposition, plasma spraying, electrodeposition and the like. In an alternative and inverse process, a membrane material, can be deposited on an existing porous electrode structure. These two processes can be used either separately, or, in combination to produce a trilayer structure.
The aforesaid herein defined cell stacks are more preferred wherein said anolyte circulation chamber has a lower portion defining an inverted triangle having an apex defining an anolyte entry port in communication with said anolyte flow inlet channel, and an upper portion defining a triangle having an apex defining an anolyte exit port in communication with said spent anolyte channel; and said catholyte circulation chamber has a lower portion defining an inverted triangle having an apex defining a catholyte entry port in communication with said catholyte flow inlet channel, and an upper portion defining a triangle having an apex defining a catholyte exit port in communication with said spent catholyte channels.
More preferably, the cell stack as hereinabove defined has the anolyte entry port central of the frame; the anolyte exit port is adjacent a first periphery of the frame; the catholyte entry port is central of the frame; and the catholyte exit port is adjacent the periphery remote from the first periphery.
The cell stack as previously defined further comprises a plurality of compressible sealing members disposed between adjacent cell members selected from the anode, the cathode, the frame and the cell wall, at the peripheries thereof and adjacent the anolyte and catholyte flow inlet channels and the spent anolyte and catholyte channels which are not facing the upper portion defining a triangle having an apex defining a catholyte exit port or anolyte exit port channel. It should be noted that the offset pairs of seals at the peripheries contains the large pressure differential with the exterior while the seals at the inlet manifolds contain a small pressure differential of about 1 psi to allow differential flow rates in the anolyte and catholyte flow channels and the seals at the spent electrolyte ports have essentially no pressure differential to contain but must prevent electrolyte from mixing and contaminating the two product streams. With regard to the spent electrolyte ports, with the preferred design having thin foil electrodes, it is not possible to provide a seal against the unsupported electrodes which are facing the upper portion of the frame member defining a triangle having an apex defining a catholyte exit port or anolyte exit port channel. Since there is essentially zero pressure drop between the circulation chambers, it is only necessary to prevent the electrolyte from flowing behind the electrode to the opposing electrolyte circulation chamber by providing one seal at the exit ports per circulation frame member. This seal is easily achieved on the side of the circulation frame member which does not have the open triangle exhaust port. Thus only one half of the exit ports are sealed and the seals alternate between one outlet. In another configuration (bipolar) described later, this alternating seal configuration is not sufficient for preventing electrolyte mixing and a stiffer electrode must be used to allow for a seal on one side of the electrode at the exit port.
Most preferably, the compressible sealing members are o-rings, and the cell members have portions defining o-ring receiving recesses.
The frames and cell end walls are most preferably formed of a structural plastics material. The anodes and cathodes are most preferably in the form a metallic foil or the like having a thickness, preferably selected from 0.05-0.10 mm. In the case of a bipolar cell stack design, hereinafter described, the electrodes have a sufficient stiffness to hold a seal with an o-ring. Thus, stiffer electrodes will be preferred in that configuration.
Although not limiting, the invention is particularly of benefit in electrochemical processes that produce one or more gaseous products, such as chlorine, hydrogen and oxygen, the latter two from the electrolysis of aqueous potassium hydroxide electrolyte solutions, particularly in a monopolar filter press structural arrangement.
In a further aspect, the invention provides an improved process for providing hydrogen from a monopolar electrolytic cell having cell walls under an external pressure; anolyte solution having an anolyte liquid level; catholyte solution having a catholyte liquid level; the process comprising generating oxygen at an oxygen pressure within the cell above the anolyte; generating hydrogen at a hydrogen pressure within the cell above said catholyte; the improvement wherein each of the oxygen pressure and the hydrogen pressures provide a positive pressure differential greater than the external pressure.
The external pressure is provided most generally by air, and at an ambient pressure of 1 atmosphere.
The positive pressure is readily attainable within the cell up to about 8 atmospheres, but a practical pressure is preferably selected from 2-6 absolute atmospheres.
The production of hydrogen from prior art monopolar cell stacks has been limited to current densities of less than about 500 mA/cm2 at steady state operation. The primary problem is that at high current density the volume of gas produced at the electrode surfaces becomes so great that cell resistance rises dramatically, liquid contact with the electrode surface is reduced and parts of the electrode may cease to function, unsteady liquid and gas flows develop and energy efficiency decreases dramatically. Resistive heating exacerbates the problems causing electrode heating and damage to cell components which renders the cell dangerous to operate.
A multicell stack according to the invention avoids these problems and allows current densities of greater than 750 mA/cm2 to be run at steady state almost indefinitely (100""s of hours). Surprisingly, these high current densities have been run with smooth, non-activated planar electrodes which are in thin foil form and with xe2x80x9czero gapxe2x80x9d cells (5.25 mm) and plastic frame members. It would be expected that at high current density ( greater than 500 mA/cm2) resistive heating in the thin foil electrodes and from the resistance in the electrolyte due to gas formation, would cause this type of cell to fail. In fact, this cell has been run for hundreds of hours at current densities up to 600 mA/cm2 without damage. The operation clearly demonstrates that the resistance buildup at high current density has been overcome. A larger power supply and thicker electrodes would allow even higher current densities to be run.
The accomplishment means that small electrolysis cells using this design are able to produce large amounts of hydrogen required for industrial regenerative and personal fuel appliance applications and space limited configurations. A side benefit is that pressurized hydrogen is available for direct storage in a vessel or media, such as metal or chemical hydride or as an economical source of pre-pressurized hydrogen for mechanical or electrical compressors for high-pressure use.
The invention is of particular, but not limiting, value in monopolar electrolyte cell stacks, and, accordingly, in a further aspect the invention provides a monopolar electrolytic cell stack having a cell stack as hereinabove defined wherein said stack walls are subjectable to a cell stack external pressure; said anode operably produces oxygen at an oxygen pressure within said anolyte chamber; said catholyte operably produces hydrogen at a hydrogen pressure within said catholyte chamber; means for providing each of said oxygen pressure and said hydrogen pressure with a positive pressure differential greater than said cell stack external pressure; and said membrane separator frames and said impermeable cell end walls as formed of a structural plastics material.
In one embodiment, the cell stack according to the invention, is based on the monopolar electrolysis cell stack design having the anodes and cathodes in a folded configuration known as a xe2x80x9cdouble electrode platexe2x80x9d, but of relatively very narrow thickness in the form of a metal foil. The cell containment is by means of a thin polymer plate of xe2x80x9cNoyelxe2x80x9d(copyright) or like engineering plastic having an electrolyte slot and flow channels for both liquid inlet and gas outlet. A diaphragm or membrane is used to separate anolyte and catholyte. Sealing of the electrolyte within the cell is achieved by means of O-rings in grooves set in the plate to provide a leak free condition at all operating values of current. The electrolyte is pumped into the cells through flow constricted channels in each cell compartment.
The high current operation is achieved by valving off the oxygen and hydrogen outlet channels until a pressure of up to 6 atmospheres is achieved. In this mode, current densities of up to 1 A/cm2 can be run at steady operation without damaging resistive heating effects. Without valving, the cell could not be run at more than 250 mA/cm2 before excessive gassing caused unstable operation and damaging resistive heating. Prior art cells operating at steady state at 1 A/cm2 with smooth planar electrodes are known, but do not have the advantages of the unitary electrolyte circulation and membrane frame according to the present invention.
The present invention provides, in one aspect, a process and apparatus for producing hydrogen gas and oxygen gas at an elevated pressure by electrolysis in an alkaline aqueous solution. Maintaining the hydrogen pressure above the catholyte liquid level within the cell offers the following advantages.
1. The drastic reduction in the volume of evolved gas lowers the electrical resistance of the mixture of electrolyte and gas bubbles within the cell compartment to produce a higher energy efficiency.
2. The smaller volume of gas also allows operation at higher current densities than is the case at lower pressures where the large volume of gas would form plugs within the cell compartment resulting in unstable operation or preventing electrolysis altogether.
3. The rate of flow of electrolyte required through the cell is much less. This decreases the size of the electrolyte channels required, the capacity of the pump, and the erosion and wear on all components of the cell caused by the high linear velocity of flowing electrolyte.
4. Separation of gas and liquid is easier since the volumetric flow rates of electrolyte and especially of the gases are much lower.
5. The hydrogen gas contains much less water vapour that may require subsequent removal.
6. Much less heat is lost from the stack due to the smaller amount of water that is evaporated.
7. Electrolysis can be carried out at temperatures above 100xc2x0 C. for greater energy efficiency.
8. Pressurized hydrogen is directly available for moderate pressure applications. Alternatively, the pressure can easily be raised further using a single-stage compressor that is much cheaper to buy and operate than the compressor required if the same amount of hydrogen was at atmospheric pressure.
The pressurized cell stack when pressurized up to 8 atmospheres is substantially constructed of a polymeric material, such as, for example, xe2x80x9cNOYELxe2x80x9d(trademark) structural plastics material.
The stack, in one embodiment, is essentially supported only by the cell end walls and wherein the polymer frames are designed with sufficient width at the top, bottom and sides and in connection with the electrode members as to withstand the internal gas pressures without other internal support and metallic end stack members. In an alternative embodiment, internal support is provided by the frame members and not the cell end walls, which may be in the form of a non-conductive, electrolyte impermeable film or the like.
In the aspect of the present invention involving a cell stack capable of being operable at greater than ambient pressures, in the issue of pressure in the cell stack, there are three directions to be considered, namely, the ends, the sides and the top of the stack. The sides and top, although not quite of the same dimensions, can be considered together because they resist the pressure in the same way, i.e., by the stiffness of the structural plastics material and their contact with the electrode members. The ends resist pressure by being thick and stiff and by having, in the preferred embodiment, tie rods connecting the two metallic end plates. These ends must resist the pressure for two reasons, namely, one is simple containment and the second is to keep the plates in good contact with the O-ring seals.
The following describes, firstly, the end plate pressure requirements, and secondly the sidewall pressure issues.
At the terminal ends of the cell stack, one plate has atmospheric pressure on its outer side, while on the inside is the pressure of the cell contents. Either the end wall per se or the end wall plus additional support for the wall must be of sufficient strength to resist any pressure differential. Thus, the last electrode of the stack can be made thick enough to resist the pressure or it can be the same as all of the other internal electrodes, but being supported by a polymeric plate or metal plate of sufficient strength to resist the pressure. A first attempt used a 0.635 cm steel plate in addition to a glass-filled polyphenylene oxide backing plate but this was of insufficient stiffness to allow complete sealing to take place within the cell stack. In a preferred embodiment of the present invention, a 0.953 cm stainless steel plate was then used which was sufficient to provide good sealing. It was found that some degree of deflection of the end walls was due to the pressure of the gases and fluids within the electrolyte frame chambers pushing outward on the end and the inward force applied by tie rods holding the two end plates together.
With reference now to the cell walls and pressure, the internal pressure must also be resisted by the frame members and peripheral o-ring seals. The pressure is from the inside of the electrolyte circulation chambers to the outside atmosphere and is contained by the frame wall and seals. Surprisingly, we have found that gasket seals which are conventionally used are less preferred in preventing leakage of electrolytes even under very small internal pressures of less than one psi. Both gaskets and o-rings are made of elastomers which distort easily under stress and thus provide no structural strength for pressure resistance. In the present cell stack, the structural strength is provided by the frame walls and by their contact with the electrodes and end plate members. The tie rods thus provide the important function of compressing the seals for leak prevention but also provide structural strength by pressing the frame members and electrodes together with the end walls. With gaskets of compressible elastomer material used as structural frame members in plate and frame designs, such as by U.S. Pat. No. 6,080,290, disclosed, for example, in U.S. Pat. No. 6,080,290, issued 2000, the cell generally cannot resist internal pressures greater than a few psi because the gaskets or frame members will expand outward. If sufficient tension from the tie rods is applied to allow friction to hold the gasket or frame member in place, then the gasket or frame member becomes so compressed that their structural function is lost. In the embodiments according to the invention, it is believed that the gaskets would require so much tie rod tension to effect satisfactory sealing, that even the rigid frame members would be susceptible to cracking or compressive failure. In sharp contrast, in the preferred practice of the invention O-ring sealing provides a number of benefits and has been demonstrated to provide a solution to this problem. An O-ring, by virtue of having a much smaller contact area with the frame members, electrodes and end walls than a gasket, does not require excessive tension from tie rods. By locating the O-rings in channels, the compression forces the frame member walls to contact directly with the metal electrodes and, thus, transfers strength to the frame member. Metals, generally, have much higher strengths than polymeric materials. In addition to the strengthening aspect, the cell stack was more rigid than it would have been with a rubbery gasket sandwiched between the cell components. Furthermore, in the preferred embodiment of the invention, the O-ring is prevented from xe2x80x9cslipping outxe2x80x9d from between the electrodes and frame members by the O-ring channels. Gasket seals, particularly with slippery caustic solutions, are quite susceptible to seal xe2x80x9cbowoutxe2x80x9d if the stack pressure rises. Thus, the current embodiments provides a significant safety advantage.
To conserve linear dimensional space, i.e. to make the cell stack as short as possible, the supporting frame is preferably thin and the O-ring channels offset one from the other within the frame so as not to weaken the frame at directly opposite locations.
We have found that the dimensions of the frame and cell wall structure formed of a structural polymeric material must be of a sufficient longitudal and peripheral thickness as not to expand with pressure and cause leakage or crack.
Thus, the pressurized cell stack aspect of the present invention allows the individual walls and frames to be formed of a structurally rigid plastics material, and optionally supported by metal end plates. The stack may be, most advantageously, operated at an electrolyte solution temperature of at least up to 110xc2x0 C.
The polymer frames are designed with sufficient width at their upper, lower and side portions as to withstand the internal pressure without additional support. The membranes may be readily bonded by, for example, adhesives or heat and pressure while maintaining precise dimensions.
The most preferred embodiment comprises integration of manifolds, flow control channels, fluid distributors and collectors across the electrolyte circulation chambers and membrane within a single frame. Only four electrolyte ports, namely, anolyte and catholyte input and output sources are required in the end wall.
In most preferred embodiments, the electrodes are relatively thin, e.g. of the order of 0.05-0.1 mm. These makes the stack more compact and comprises monolithic, impermeable, single sheets of foil with no perforations that do not need a second central element, i.e. no sharp, distinct electrode frame to act as a current carrier.
In a further aspect, the invention provides a novel pressure control system of particular value in offering improved pressure regulation at elevated temperatures and relatively high current density which produces relatively high volumes of hydrogen and oxygen gases within the confines of a compact cell and cell stack according to the invention. It is a distinct advantage that such a pressure control system provides for pressure differentials of less than 12 mm water between the anolyte and catholyte circulation chambers and maintains steady this pressure differential to avoid mechanical stress on the separator membrane diaphragm and to avoid electrolyte crossover from anolyte to catholyte or vice versa.
The pressure control system of use in the present invention provides robust and steady control of pressure with a high degree of safety. In consequence of the electrolysis cell reactions, the hydrogen gas flow rate is twice that of the oxygen flow rate. The main features compensate for the differential gas volume generation within the cell, piping and gas liquid separators. This differential in flow rate is not so important in wide gap cells, but in the compact cell stacks according to the present invention, the gas and electrolyte flows experience significant hydraulic resistance due to the narrow gap between walls and constricted flow at exit apertures which must fit within the compact stack structure. Increase in operating temperature, which is important in reducing voltage loss, further requires good pressure control and gas volume management, since the volumes of gas rise directly proportional to the temperature in the cell. Passive cell design control features include a reduction of the electrolyte flow to the anolyte compartment by one-half of the catholyte flow as described hereinafter, in order to achieve the same gas to liquid fraction in both circulating chambers. This provides the same fluid density in both sides, which is important for achieving the same constant liquid level head in the gas liquid separators. The outlet manifolds and tubing are kept as large as possible to minimize friction. The tubing length from the cell stack to the gas liquid separator for the catholyte and hydrogen gas is reduced relative to the anolyte and oxygen gas to compensate for the higher electrolyte plus hydrogen gas flow rate.
The level of the liquid in the, respective, gas liquid separator is used as the first indicator of the pressure differential in the cell stack. The pressure control of the system must allow the liquid levels to stay equal or within some small level of tolerance supported by the membrane. Although electronic control systems may be used to adjust pressure regulators a more direct method is, preferred, involving a matched pair of back pressure regulatorsxe2x80x94one for oxygen and one for hydrogen. These back pressure regulators are controlled by a common, single compressed gas pressure source. This common single compressed gas always provides the same release pressure to both regulators automatically regardless of the control pressure. With this device the primary pressure balance control is assured. Operation of the cells with this configuration shows exceptionally stable pressure balance during many hours of operation, even at high current density  greater than 600 mA/cm2 and temperatures up to 100xc2x0 C. This control of pressure balance is achieved without the continuous control adjustment that is required by electronic control systems. Furthermore, no sensor input is required for this control and, thus, adds a high level of security to the system.
With all control systems, there is a long term drift that must be addressed. In much of the testing work, occasional manual adjustment of the regulator was required to keep the system in balance. In industrial practice, manual adjustment is not desirable for continuous operation.
Thus, in a further aspect, the invention provides an improved process for providing hydrogen and oxygen gases from an electrolytic cell stack having
a spent anolyte solution having an anolyte liquid level and oxygen gas above said anolyte liquid level;
a spent catholyte solution having a catholyte liquid level and hydrogen gas above said catholyte liquid level;
the improvement comprising
detecting at least one of said anolyte and said catholyte liquid levels;
releasing said oxygen gas from above said anolyte level when said catholyte liquid level is detected;
or releasing said hydrogen gas from above said catholyte level when said anolyte liquid level is detected;
wherein said detection of said anolyte level comprises irradiating said anolyte liquid level with incident infrared radiation at an angle to effect scattering of said radiation; and
wherein said detection of said catholyte level comprises irradiating said catholyte liquid level with incident infrared radiation at an angle to effect scattering of said radiation.
In a further aspect the invention provides an electrochemical stack as hereinabove defined further comprising:
a spent anolyte solution having an anolyte liquid level and hydrogen gas above said anolyte liquid level;
a spent catholyte solution having a catholyte liquid level and hydrogen gas above said catholyte liquid level;
means for detecting said anolyte and said catholyte liquid levels;
valve means for releasing said oxygen gas from above said anolyte level when said catholyte liquid level is detected;
means for releasing said hydrogen gas from above said catholyte level when said anolyte liquid level is detected;
wherein said detection of said anolyte level comprises means for irradiating said anolyte liquid level with incident infrared radiation at an angle to effect scattering of said radiation; and
wherein said detection of said catholyte level comprises means for irradiating said catholyte liquid level with incident infrared radiation at an angle to effect scattering of said radiation.